Electroluminescent lamps and phosphors

ABSTRACT

Electroluminescent phosphors, electroluminescent panels and lamps made with such phosphors, and a process and apparatus for treating phosphors are disclosed in which the phosphor particles are coated with a very thin coating of SiO 2 , to protect the phosphor particles from aging due to moisture intrusion. The phosphor particles are coated in a cold wall reactor by the pyrolytic decomposition of silane in the presence of heat and oxygen to a coating thickness of approximately between 0.1 and 3.0 microns. The apparatus and method of coating includes the placement of a quantity of phosphor in a cup-shaped heated reactor bowl and subjecting the particles to a temperature of about 490° C. and an atmosphere of silane and oxygen, while continuously mechanically agitating the particles with a blade arrangement in which the particles are continuously rotated and turned so as to expose the surfaces of the heated particles to the reaction atmosphere. Panels and lamps made from such phosphors may be die cut and trimmed, have an increased life as compared to panels and lamps made with untreated phosphors, and exhibit a minimum of color shift during the lifetime of the panel.

This invention relates to improved electroluminescent lamps andphosphors, and to a method of making such improved phosphors.

Conventional electroluminescent lamps have panels which are made withelectroluminescent phosphors, such as copper activated zinc sulfide,embedded in a resin layer between a pair of electrodes. Suchconventional lamps suffer from aging and degradation due to moisture,such as by the migration of water molecules into the matrix of thephosphor crystals. The aging process is accompanied by a loss inbrightness, for a given level of excitation, and a shift in color of thelamp, both in the lamp's lighted and unlighted status. As a result, ithas become necessary to go to extraordinary lengths to protect such lampand panels against moisture.

The lighted panels of conventional lamps also do not lend themselves tomanufacturing processes which include die cutting, punching,perforating, or trimming through the active phosphor layer, as suchoperations will either immediately short out the lamp, or will result ina premature loss of brightness in the vicinity of the cut, and oftenaccompanied by eventual failure of the entire lamp. Such behavior ofconventional lamps severely restrict the use of electroluminesent lampsin many commercial applications which require punching, die cutting orthe like, in a low-cost and mass-produced panel and/or in which moistureis present.

Electroluminescent lamps which lack extraordinary external protectionagainst the infusion of moisture are not only prone to suffer from lossof output, i.e., aging, but as noted above are observed to have a shifttoward the color pink where the lamp output was originally white.Further, where the natural color of such a lamp in its unlightedcondition is an overall light tan, such lamps have been observed, withaging, to take on an overall gray or black color. In many cases, such achange in color is undesirable or unacceptable.

The desirability of encapsulating electroluminescent phosphors to retardaging has been recognized. Both organic and inorganic coatings have beensuggested, with varying degrees of success. One approach, as disclosedin the patents of Allinikov, U.S. Pat. No. 4,097,776 issued June 27,1978 and Olson et al, U.S. Pat. No. 4,508,760 issued Apr. 2, 1985,includes the use of organic or polymer resin materials forencapsulation. In Allinikov, the phosphor particles are immersed in asolution of liquid crystal material and stirred, and thereafter dried toform a resin coating. In Olson et al, specific polymers are vacuumdeposited on the surface of the crystals.

Resin coated phosphors have not found general use in the manufacture ofelectroluminescent lamps since they suffer from many of the sameproblems as do conventional resin embedded particles, that is, that theresins do not fully exclude moisture and may interact with the phosphor.When such resin encapsulated particles are used as a substitute forconventional uncoated particles, they may be mixed with a resin adhesiveand applied, as by screen printing or by a blade, to a substrate in themanufacture of the lamp. The intermediate resin coating surrounding theparticles is usually no better in preventing aging than is the adhesiveor casting resin itself.

The prior art also contains a number of attempts to provide an inorganicbarrier or coating on the phosphor particles, including Piper, U.S. Pat.No. 2,944,177 issued July 5, 1960. In Piper, phosphor crystals orparticles are mixed with a glass frit, and then heated to approximately530° F. until the glass fuses, producing a phosphor and glassaggolomerate This is then cooled and crushed until the resultingparticles are sufficiently small so as to be applied as a glass coatedparticle in lieu of conventional electroluminescent phosphor grains.However, the glass fusing and crushing process of Piper has not comeinto general usage because of two principal disadvantages. First, incrushing or grinding, many of the phosphor particles themselves areruptured or broken and exposed, and are therefore subject to the normaleffects of aging Further, the process produces too much glass inrelation to the phosphor content, without close control of the thicknessof the glass deposition with respect to the phosphor particles.

Brooks, U.S. Pat. No. 3,264,133 of Aug. 2, 1966 discloses the coating ofthe phosphor particles with an inorganic coatings, such as bariumtitanate and titanium dioxide, to provide a high dielectric coating.While Brooks achieves an enhancement in brightness due to an increase indielectric constant, it is not apparent that these coatings are usefulto extend the life of the phospor or exclude moisture.

In a number of related patents, Fischer has described the aging processin zinc sulfide phosphors and provides recipes for rendering thephosphors less immune to aging and for coating the phosphors withinorganic phosphates. These patents include U.S. Pat. Nos. 4,143,297issued Mar. 6, 1979; 4,181,753 issued Jan. 1, 1980; and 4,263,339 issuedApr. 21, 1981. The aging process is described in '297 as beingaggravated by sulfur vacancies in the crystal lattice structure, whichvacancies exhibit a negative charge and the presence of which Fischerbelieves promotes the diffusion of the positively charged copper ionswithin the grains of the phosphor. Fischer further believes that thecopper out-diffuses to the surface and the electroluminescent mechanismbecomes inoperative due to this form of aging. It is also clear that theaging is accelerated by the presence of moisture and electrolysis of thezinc and copper. As an intermediate step, Fischer treats his phosphorprior to coating by immersion in molten sulfur under heat and pressurein an autoclave. In '753, the disclosure is enhanced by the suggestionthat metals may be added to the sulfur bath.

After the sulfur process, Fischer boils the treated powder in aconcentrated phosphoric acid to form an insoluble zinc phosphate skinaround each particle. The light transmissive qualities of this coatingare not disclosed. Patent '753 discloses a further intermediate step,prior to the phosphoric acid bath, of heating the sulfur treatedparticles in hydrogen peroxide to convert the zinc sulfide surface to azinc oxide surface and thereafter treating in phosphoric acid to convertthe zinc oxide to the zinc phosphate coating.

Fischer also suggests that the particles can be glass coated, and statesthat the coating "can also consist of chemically vapor-deposited glass .. . produced by pyrolytic decomposition of metal-organic vapors." Noexample is given in Fischer of the metal-organic vapors, of any processfor accomplishing the process, or of any lamp using such phosphors.

Attempts have been made to coat phosphor particles with glass, i. e.,silicon dioxide, and include the U.S. Pat. No. 3,408,223 of Shortes,issued Oct. 29, 1968. Shortes was not concerned with the coating ofphosphor particles for use in electroluminescent lamps and therefore wasnot concerned about extending the life of such a lamp or the phosphorstherein, or the exclusion of water vapors from interaction with thephosphor particles. Rather, Shortes was concerned with the manufactureof a cathode ray tube phosphor which had selectively higher electronenergy ionization thresholds, and disclosed the coating of phosphorparticles by subjecting the phosphors to a tetraethoxysilane atmosphereunder high temperature conditions, and subjecting the phosphor particlesrepeatedly to such atmosphere by recirculating the atmosphere and/or thephosphor particles therethrough so as to provide a silicon dioxidecoating. Shortes contains no disclosure of the thickness or character ofthe coating, or of the efficacy of the use of such a treated phosphorparticle in an electroluminescent environment.

U.S. Defense Department Technical Report AFFDL-TR-68-103 "Improving thePerformance of Electroluminescent Lamps at Elevated Temperatures," July1968 by Thompson et al, published by United States Air Force FlightDynamics Laboratory, ASFC, Wright-Patterson Air Force Base, Ohio,discloses the coating of electroluminescent particles with variousrefractory materials including silicon dioxide, titanium dioxide, andberyillium oxide, among others. All of the coatings were applied by thepyrolysis of chemical vapors at atmospheric pressure in a heatedfluidized bed reactor. The silicon dioxide coatings were applied by thedecomposition of tetraethyl orthosilicate Si(OC₂ H₅)₄ or silicontetrachloride SiCl₄, with reactor temperature of 400° C. There is nomention in this respect of silicon coated phosphors used in anelectroluminescent lamp. Rather, the authors concentrated primarily onthe use of titanium and beryllium coated phosphors in making, and thentesting electroluminescent lamps at very high operating temperatures.The titanium coated particles tended to fuse together or cluster intogroups of coated particles, and it was difficult to maintain the desiredphosphor population in a lamp, apparently due to the shape of theparticles and the quantity of coating included. Accordingly, the overalllamp brightness was reduced due to the reduced phosphor particlepopulations as compared to a conventional lamp using uncoated phosphors.The authors, however, indicated that the silicon dioxide coated zincsulfide phosphor was given an accelerated water vapor resistance test,not otherwise described, and indicated that the material "looked like itshowed promise."

SUMMARY OF THE INVENTION

This invention relates particularly to an electroluminescent lampincorporating phosphor particles which are coated with a thin coating ofsilicon dioxide, and to such phosphors and the method of making thesame.

Applicants have discovered that an electroluminescent lamp made with aphosphor in which the individual phosphor particles are coated with avery thin coating of silicon dioxide, provides surprising and unexpectedresults. Such a lamp has been found to have aging characteristics whichclosely parallel those of fully incased lamps. In addition, such lampsaccording to this invention do not initially suffer any substantial lossin brightness by reason of the presence of the coating on the phosphorparticles, and do not exhibit the characteristic color shifts with agingwhich have been observed in conventional lamps. Also, the lamps, and thepanels from which such lamps are made according to this invention, maybe cut, punched, or otherwise severed through the active phosphorcoating with minimal darkening, discoloration or loss of brightness atthe exposed edges.

The invention also includes the method and apparatus by whichelectroluminescent phosphors are treated, by the application of a thinhomogeneous coating of silicon dioxide to the particles, in the natureof one micron or less in thickness. A cold wall reactor is disclosed,which provides for the heating of phoshor particles to a temperaturesufficient to decompose silane in the presence of oxygen, whileproviding for the stirring and agitation of the particles so that allsides are uniformly coated.

More particularly, the coating method employed by the coating apparatusof this invention includes the steps of heating the phosphor particlesto be coated and while so heated, subjecting the particles to anatmosphere of silane and oxygen such as to cause silicon dioxide to bedirectly vapor-deposited uniformly over the surfaces of the phosphorparticles. Examination confirms the formation of a thin clear glasscoating fully surrounding the individual particles. To enhance thecoating process, an electrostatic charge may be applied to the gas ionsor between the gas ions and the particles to enchance migration of thegas ions and their combination on the surfaces of the phosphorparticles. More particularly, the phosphor to be treated is subject to acontrolled environment of SiH₄ and O₂, at an elevated temperature abovethat required to decompose the SiH₄, such as about 480° C., whilecausing the particles to be moved, tumbled, or stirred, and preferablywhile directing the gases to the particles' surfaces so as to formcomplete glass coatings on the particles.

The improved phosphor, according to this invention, exhibits a clearcoating of silicon dioxide as far as discernible, in a single thin layerbetween 0.1 and 3.0 microns in thickness, and preferably between 0.4 and1.0 microns. The thickness of the layer does not vary materially betweenphosphor particles of different sizes. Since the silicon dioxide layeris formed in a continuous process, the layer on the particles ishomogeneous and free of demarcation lines or changes in crystallinestructure in relation to its thickness. This single, homogeneous layerof silicon dioxide is attributed to the cold wall reactor and process ofthis application, and by the use of silane as the silicon donatingcompound, and oxygen, in the process.

It is accordingly an important object of this invention to provide anelectroluminescent phosphor and a lamp employing such phosphors, highlyresistant to aging due to moisture or water molecule intrusion.

Another object of this invention is the provision of a lamp, as outlinedabove, characterized by uniform light output throughout the servicelife, with a minimum of color shift either in the energized orunenergized condition.

A still further object of the invention is the provision of a flexibleelectroluminescent lamp including a panel section which may be die-cut,trimmed and punched, without any substantial loss of light output ordarkening around the cut edges, and without premature failure of thelamp.

A further object of the invention is the provision of a method ofapplying a uniformly thin and uniformly distributed silicon dioxidecoating to electroluminescent particles for use in lamps.

Another object of the invention is the provision of a cold wall reactorapparatus useful in the controlled application of silicon dioxidecoating to phosphors.

A further object of the invention is the method of coatingelectroluminescent phosphors and phosphors so coated by the pyroliticdecomposition of silane in the presence of an oxygen carrier.

These and other objects and advantges of the invention will be apparentfrom the following description, the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a sectional view of a cold wall reactor for coating phosphorparticles with SiO₂ in accordance with this invention;

FIG. 2 is an enlarged fragmentary sectional view through the reactionvessel, illustrating the stirring apparatus;

FIG. 3 is a view of the reaction vessel looking down into thephosphor-containing bowl;

FIG. 4 is an enlarged cross-sectional view of the coated particles;

FIG. 5 is a diagrammatic cross section through a lamp panel inaccordance with this invention;

FIG. 6 is an enlarged fragmentary sectional view showing the attachmentof an electrode to such lamp;

FIG. 7 is a time line diagram of a humidity test; and

FIGS. 8-9 are performance graphs of test lamps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Phosphor processed in accordance with this invention is coated withsilicon dioxide by chemical vapor deposition in a cold wall reactorillustrated in FIGS. 1-4. The reactor 10 has a generally cylindricalcontainer wall 12 closed by a lid 13 and a bottom 14. The wall 12 of thereactor is chilled by a cooling coil 15, which coil has an inlet 16 atthe top and an outlet at the bottom. The cooling coil may be embedded inthe wall or may be formed closely to the wall interior as shown orwrapped about the exterior of the wall. The coil 15 cools the wall tomaintain a temperature substantially below that in which the silica isdeposited on the electroluminescent phosphor crystals.

The cold wall reactor 10 has a base 20 mounted on the floor 14 by atripode consisting of three adjusting screws 22, shown for the purposeof illustration as being in one line, but which actually occupy 120°relative positions, by means of which the base may be adjusted withrespect to the floor 14. The base 20 supports an annular bowl retainer25. The cup retainer 25 is heated by an electrical resistance heater 26with leads 27 and 28 extending therefrom.

The central portion of the retainer 25 defines a cylindrical openingwhich receives a high temperature reactor bowl 30 (FIG. 2). The base ofthe bowl 30 is generally flat with vertical sides. The bowl 30 hasembedded therein a thermocouple 32 with a pair of wires 33 thereto. Thereactor bowl 30 receives a quantity of phosphor to be treated to anelevated temperature. A combined stirring, scraping and smoothing bladearrangement illustrated generally at 35 is received within the bowl forscraping, lifting, spreading and smoothing the heated electroluminescentphosphor 50 within the bowl 30, as rotated by an adjustable stirring rod36. The lower end of the rod 36 is received within a conical centraldepression 38 (FIG. 2), formed in the bowl, forming a centering bearingfor the rod 36. The blade arrangement 35 is mounted on the lower end ofthe stirring rod 36. As shown in FIG. 1, the upper end of the rod isadjustably driven by a coupler 37 to a 0.5 rpm DC drive motor 40,mounted on legs 42 on the cover 13.

With further reference to FIGS. 2 and 3, the blade arrangement 35includes sheet metal lifting and scraping blade 44 and a smoothing blade45. The blades 44 and 45 extend generally radially outwardly from therod 36, and are welded, such as by spot welding to opposite sides of therod, as shown in FIGS. 2 and 3. The lifting and scraper blade 44 definesan angulated mixing paddle, with a lower flat edge 44 which rides alongthe flat bottom of the reactor bowl 30. The block 44 has a radial lengthextending to the vertical walls of the bowl, and firm about a 30° angleto the floor. The blade 44 runs submerged in the phosphor and itsinclined surface gently engages the phosphor 50 in the bowl and causesthe phosphor to be lifted along the surface, turned, and depositedbehind it in its direction of travel, as illustrated in FIG. 2.

The opposed smoother blade 45 is also paddle-shaped and has a lowerstraight edge 45' in spaced relation to the floor of the bowl 30. Theblade 45 follows the blade 44 by approximately one-half turn or 180°,and smooths the upper surface 52 of the phosphor 50 after it has beenturned by the lifting and scraping blade 44.

A third blade indicated generally at 60 is mounted on an arm 61 forrotation with the rod 36 and has an outer paddle-like or leaf-like end62, as best shown in FIG. 3. The blade end 62 engages phosphor in thecorners of the bowl and along the bowl wall, and moves it in a pile awayfrom the wall for engagement and leveling by the following blade 45.

The cold wall reactor 10 includes a pair of gas diffuser nozzles, shownin FIG. 1. A first nozzle 63 is mounted on the end of a depending tube64, and a second nozzle 65 is mounted on the end of a depending tube 66.One or more additional inlet tubes (not shown) may be mounted in thecover such as for applying other gasses used as dopants.

The tubes 64 and 66 depend through the cover 13 and extend partiallyinto the interior of the reactor 10. The nozzles 63 and 65 are pointeddownwardly toward the bowl 30, and one of these nozzles, such as thenozzle 63, emits a nitrogen-silane mixture while the other nozzle 65emits oxygen. The tube 64 is connected to means (not shown) by whichsilane gas (SiH₄) is blended with nitrogen, in an approximate ratio of 1to 2% silane to 99 to 98% nitrogen. An amount of oxygen through thenozzle 65 is sufficient to provide a Molar concentration of 3 to 1 orgreater oxygen to silane. If desired, a small impeller 70, driven by amotor 72, may be employed to assure circulation of the products withinthe reactor.

In use, a crystalline phosphor, of the general kind described below, isdeposited in the bowl 30 approximately to the level of the blade 45. Fora five-inch diameter bowl, approximately 15 grams of phosphor crystalsare placed in the bowl. Pure nitrogen is then introduced through thenozzle 63 to flood the interior and to expel any other gases, through anoutlet tube 79 in the cover 13. At the same time, the heating element 26is turned on and a temperature controller permits the reactor bowl toheat up to 100° C., to allow the temperature to stabilize. The motor 40may now be turned on. By viewing the process through a top-viewingwindow 72, one can observe that the phosphor crystals are being smoothedby the blade 45 and turned by the blade 44. If the level of phosphorappears uneven, the level of the bowl may be adjusted by the screws 22.

Following this, the temperature may be increased to approximately 200°C., allowed to stabiize, and then increased to 400° C. and stabilized.After such stabilization, silane gas is blended with the nitrogen in theratios defined above and brought in through inlet tube 64 and nozzle 63,and oxygen is admitted through the nozzle 65, and the reactortemperature is raised to 450° C., allowed to stabilize, and thenincreased to 490° C. As an example, satisfactory results have beenobtained with flow rates of 1.29 liters per minute of a silane nitrogenmixture and 1.85 liters per minute of oxygen, in which the ratio ofsilane to nitrogen was 1% silane, 99% nitrogen.

Cooling water is forced through the cooling coils 15 from the inlet 16to the outlet 17, to maintain the wall of the reactor 12 substantiallycooler than the quantity of phosphor 50 and the bowl 30. This preventsunwanted reactions on these surfaces and encourages the reaction to takeplace on the phosphor itself.

The circulating impeller 70 and motor 72 assures that the gases arecirculated to prevent stratification of temperature. It has been foundthat if the temperature at any one location becomes too high, a silicadust tends to form which acts as a contaminate to the phosphor. On theother hand, if the gases become too cold, they will unduly chill theexposed layer of phosphor on the surface of the bowl 30 andsubstantially increase the reaction time. The silane at the reactiontemperature 490° C. undergoes a pyrolytic decomposition to form puresilica and hydrogen. The amount of hydrogen produced by the reaction isvery small, and due to the large quantity of nitrogen present, it is notas hazard.

The slow rotation of the stirring rod 36 causes the blade 44 to scrapethe heated phosphor 50 from the bottom of the bowl 30 and gently liftthe same over the top of the blade, where it freefalls behind the blade,thus gently turning the phosphor particles to expose new heatedparticles to the reaction gases at the surface where the major portionof the coating process occurs. The particles are heated by contact withthe bowl surfaces then turned and brought to the surface for reaction bythe action of the blades. The blade 44 thus runs substantially submergedin the pool of phosphor particles 50, as shown by the broken outlineview in FIG. 3, while the smoothing blade 45 has its bottom edge 45'spaced above the bottom of the bowl, and smooths the phosphor at thesurface, also tending to rotate or turn the top phosphor particles as itpasses. The blade 45 also levels the row of phosphor made by the paddleend 62 of the arm 60. The combined actions of the blade arrangement 35assures that each particle is smoothly and completely encapsulated andcovered as shown in FIG. 4, and further resists the tendency of thephosphor particles to group together or form in clumps.

The reactor 10 is allowed to run for approximately 1.5 to 2.5 hours atthe final temperature, and then is shut down. The phosphor crystals, asmagnified approximately 1200 times are shown in FIG. 4 as having auniform coating 80 of silicon dioxide, which coating is fully continuousabout each particle with very little evidence of clumping of particles.Further, the coating 80 is approximately the same thickness for each ofthe individual particles 50a, b, c, and d, regardless of the shape orsize of the particles.

Good results have been obtained by the use of a blended white phosphor,primarily copper or magnesiumactivated zinc sulfide, in accordance withSylvania Specification No. 830. This phosphor is found to have a sizedistribution as follows: 5% exceeds 39.5 microns; 50% exceeds 27microns; 95% exceeds 14.6 microns. In general, the phosphor particleshave a size distribution in which about 90% of the particles are between14 and 62 microns in size, as measured by a Coulter counter.

The chemical vapor deposition provides a uniformly thin continuouslayer, relatively constant thickness with a minimum thickness in theorder of 0.1 micron and a maximum thickness in the order of 3 microns.Preferably, the coating thickness is between 0.4 and 1.0 micron. Such anextremely thin coating, in relation to the size of the phosphorparticle, permits the highest possible electrical field across theparticles and the presence of the coating does not adversely affect orsubstantially reduce the amount of active phosphor which may be appliedto any given electroluminescent panel.

The method and apparatus also permit relatively low temperaturedeposition which is not harmful to the electroluminescent crystals. Theapparatus is of simple construction and relatively easy to operate.Further, the crystals of the phosphor are not in constant motion, andare covered or coated in essentially a single process.

Test panels were made employing phosphor which has been conditioned inaccordance with this invention and compared with identical test panelsusing uncoated phosphor. The test panels were compared to panels whichwere completely sealed in polychlorotrifluoroethylene, such as "Aclar",available from Allied Chemical Company, after the panel was completed,for total exclusion of moisture.

In each of the test panels, a base resin was prepared for the phosphorlayer, for the dielectric layer, for the electrode layer, and for aprotective overlayer of the same resin material, namely a polyester baseconsisting of approximately 50% cyclohexanone, 16.7% diethylene glycolmonobutyl ether acetate and 33.3% polyester adhesive 49001 of DaytonChemicals Division. This base resin was then mixed approximately 72%processed phosphor to 27% resin for the phosphor layer, 55% bariumtitanate for the dielectric layer, and 71% flaked silver for theconductive layer. 100% resin was used for the protective layer. Each ofthe layers was activated by approximately 0.4% to approximately 1.6%Adcoat Catalyst F of Morton-Thiokol, Inc. to reduce curing time. Allpercentages are by weight.

Lamps constructed for the purpose of the evaluation of the conditionedphosphor in accordance with this invention are illustrated in FIG. 5.Each of the test lamps was constructed on a clear flexible base 100 ofPET (biaxially oriented polyethylene terephthalate) material, to whichhad been applied a transparent electrode which is diagrammaticallyillustrated at 102. The transparent electrode is a metalized, vapordeposited indium-tin-oxide conductive coating having a resistance in theorder of 175±25 ohms per square, and is shown in exaggerated thickness.

The base 100 with electrode 102 thereon was heat treated at 121° C. for30 minutes, to stabilize the film to prevent warping during theink-drying process.

The phosphor resin layer 105 was applied to the base 100 on theelectrode 102 by screen printing after the phosphor had been blendedwith the resin, as identified above, by thoroughly mixing with a spatulato wet out all of the phosphor particles. The resulting phosphor ink hada viscous cream-like consistency. The phosphor ink was applied through a157-mesh screen and dried at 110° C. for thirty minutes.

The barium titanate layer 110 was prepared in accordance with the aboveformula, in which the resin was added to the powder followed by mixingwith a spatula to wet all particles, followed by mixing in a blender athigh speed for 10 minutes, and then rolled overnight. Prior to mixingwith the resin, the barium titanate powder was sieved and dried at 250°C., to eliminate all moisture prior to blending with the resin. Thebarium titanate layer 110 was applied as two layers, one directly on topof the other, through a 95-mesh screen. Each layer was dried at 110° C.for 50 minutes.

The silver electrode layer 112 was prepared in accordance with theabove-defined formula by mixing with the resin with a spatula to wet allparticles, then mixed in a high speed blender for 10 minutes and rolledovernight, in the same manner as that of the layer 110. The silverelectrode layer was applied through a 330-mesh screen as a single layerand dried for 40 minutes at 110° C.

Finally, a clear protective layer 115 of resin was applied to the silverlayer through a 195 mesh screen, and dried for one hour at 110° C. Alltest lamps were die-cut to 2 inches by 4 inches in size.

The lead attachments to the test panels were made as shown in FIG. 6.Wire mesh power leads such as the lead 120 was attached to the silverelectrode 112 of a test panel 125 by means of a conductive transferadhesive 124, and laminated in place. Electrode attachment may also bemade by use of conductive epoxy or adhesive. The connection is protectedby a layer 128 of Tedlar tape, and applied to the wire mesh leads 120,and to the panel edge, for mechanical support. A similar lead attachmentwas used for the transparent electrode 102.

Performance tests were made on the test lamps constructed, as definedabove, with coated and uncoated phosphors, and compared against theperformance of a totally "Aclar" encapsulated electroluminescent lamp.The lamps were tested under three different environmental testconditions: (A) Humidity, with Lamps Operating; (B) Humidity with LampsNot Operating; and (C) Standard Laboratory Conditions (SLC) with LampsOperating. Two each of the test lamps were used in each test and theresults averaged between them. All electroluminescent lamps, at thebeginning of each test, were energized to 12 foot Lamberts. The voltageand frequency operating condition for each type of lamp was as follows:

    ______________________________________                                        Lamp Type      Voltage (VAC)                                                                              Frequency (Hz)                                    ______________________________________                                        Unprocessed phosphor                                                                         140          700                                               Processed phosphor                                                                           200          900                                               "Aclar" Encased                                                                              100          400                                               ______________________________________                                    

A diagram (FIG. 7) of the Humidity with Lamps Operating (A) is shownbelow. The lamps were operated for nineteen hours and off for fivehours. The temperature was 43° C. and the humidity was 95% RH except forone hour where the temperature (22° C.) and humidity (30% RH) werelowered to allow access into the chamber for light readings.

The Humidity with Lamps Not Operating test (B) was performed under thesame temperature and humidity conditions as (A) above. The lamps werelit during the one hour test at 22° C. temperature and 30% RH to obtainlight readings.

The SLC test (C) was conducted with lamps operating continuously. Thetemperature and humidity under SLC were approximately 22°-24° C. and40%-60% RH, respectively.

The cumulative results for the three lamp types versus the three testconditions is shown in Table 1. The performance graphs for these areshown in FIGS. 8 and 9.

Test C Humidity--Non-operating

In this test, all lamps performed equally well, and did not experience acolor change, edge darkening or brightness loss.

Test A (FIG. 8) Humidity--Operating

The "Aclar" lamp was about equal to the lamp with processed phosphor andsuperior to the lamp with unprocessed phosphor.

The processed phosphor lamp and the "Aclar" lamp remained white incolor, while the unprocessed phosphor lamp turned pink.

Edge and panel darkening was very pronounced with the unprocessedphosphor panel. The "Aclar" lamp did not show this phenomenon and theprocessed phosphor lamp exhibited only slight darkening.

All lamps lost brightness during the humidity test cycle. After 480hours, the unprocessed phosphor, processed phosphor and "Aclar" lampshad retained 20, 30 and 35%, respectively, of their initial brightness.

Test B (FIG. 9) SLC--Operating

All lamps retained their white operating color. Only the unprocessedphosphor lamp showed the edge and panel darkening phenomenon. Thebrightness retention of the processed phosphor lamp and the "Aclar" lampwere essentially equivalent at 51% and 54%, respectively. Theunprocessed phosphor lamp retained 43% of its original brightness value.

                                      TABLE 1                                     __________________________________________________________________________            Humidity   Humidity   SLC                                                     Non-Operating                                                                            Operating  Operating                                       LAMP    A   B  C   A   B   C  A   B  C                                        __________________________________________________________________________    Unprocessed                                                                   Phosphor                                                                              White                                                                             No 100 Pink                                                                              Yes 20 White                                                                             Yes                                                                              43                                       Processed                                                                             Blue           v.                                                     Phosphor                                                                              White                                                                             No 100 White                                                                             slight                                                                            30 White                                                                             No 51                                       "Aclar" White                                                                             No 100 White                                                                             No  35 White                                                                             No 54                                       __________________________________________________________________________     A = panel color after 480 hours                                               B = edge darkening                                                            C = percentage brightness retained after 480 hours                       

Significantly, the lamps which contained phosphor coated in accordancewith this invention performed approximately equal to the "Aclar" encasedlamp. The lamps retained their original desirable white color, edgedarkening was negligible, and the lamps further retained their originaloverall light tan color in the unlighted condition, while lamps usingthe unprocessed or uncoated phosphor showed a shift to gray color in theunlighted condition. Table No. 2, below, shows the results of colorchange in the X and Y coordinates based on the standard CIE chromoticitychart and system of coordinates when lamps incorporating coated orprocessed phosphor were operated under standard laboratory conditionsand under conditions of high humidity, as previously defined in FIGS. 9and 8, respectively, and as compared with test lamps made with untreatedphosphor under the same conditions. It will be seen by reference toTable 2 that the lamps which contained phosphor coated in accordancewith this invention exhibited less color shift than did lamps employinguntreated phosphor.

                  TABLE 2                                                         ______________________________________                                                    SCL        Humidity                                                           Ohrs  336 hrs  Ohrs    1000 hrs                                   ______________________________________                                        Processed Phosphor                                                            X             0.313   0.333    0.376 0.459                                    Y             0.361   0.375    0.346 0.426                                    Unprocessed Phosphor                                                          X             0.345   0.379    0.345 0.471                                    Y             0.352   0.368    0.359 0.527                                    ______________________________________                                    

An important advantage of lamps made employing the phosphor processedaccording to this invention resides in the fact that the lamps may betrimmed, die-cut or punched, after manufacture, through the operativelayers, with only minimal edge darkening, even under the severe humidityand temperature conditions of test A, FIG. 8.

While the method and product herein described, and the form of apparatusfor carrying this method into effect, constitute preferred embodimentsof this invention, it is to be understood that the invention is notlimited to this precise method, product and form of apparatus, and thatchanges may be made therein without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. An improved long-life, flexibleelectroluminescent lamp which may be die cut or served by cutting ortrimming through all of the active layers, including the phosphor layer,and which maintains its brightness up to a cut or severed edgesubstantially for the life of the lamp, and which exhibits a minimum ofcolor shift during use, comprising:a base formed of temperaturestabilized, flexible film material, a transparent electrode on saidbase, a phosphor layer on said transparent electrode, said phosphorlayer including phosphor particles in a size range in which about 90% ofthe particles are between about 14 and 62 microns in size, each of saidparticles being coated with an encapsulating layer of silicon dioxideformed thereon as the product of pyrolytic decomposition of silane inthe presence of oxygen at the surface of a heated particle, saidcontaining on each said particle being of substantially uniformthickness in the order of approximately between 0.1 and 3.0 microns,said coated phosphor particles being dispersed in a resin carrier, adielectric layer applied to said phosphor layer, and a back electrodeapplied to said dielectric layer.
 2. The electroluminescent lamp ofclaim 1 in which said silicon dioxide layer is formed as a homogeneoussingle thickness conforming to the shape of the associated phosphorparticle.
 3. The lamp of claim 1 in which said coating is between 0.4and 1.0 micron in thickness.
 4. An improved flexible electroluminescentlamp, comprising:a based formed of temperature stabilized flexible film,a transparent electrode on one surface of said base, a phosphor layer onsaid transparent electrode, said phosphor layer having discreteparticles of phosphor in a cured resin binder, each of said particlesincluding a discrete encapsulating coating of clear silicon dioxide,each said coating having a substantially uniform thickness, and eachsaid particle having a coating of substantially the same thickness asthe other particles in the order of 0.1 to 3.0 microns, said coatingbeing formed on said particles as the product of pyrolytic decompositionof silane in the presence of oxygen at the surface of a heated particle,a dielectric layer on said phosphor layer, and a back electrode layer onsaid dielectric layer.
 5. The lamp of claim 4 in which said coating isbetween 0.4 and 1.0 micron in thickness.
 6. The flexibleelectroluminescent lamp of claim 4 in which said phosphor particles havea size distribution such that about 90% thereof are between 14 and 62microns in size.